Weird lp gains and losses

I have 59% win rate on my alt account platinum 3. Last 4 games, I lost a game -15lp, then I won +21, won again +19, lost -19. In all games, both sides' team mmr was equal. I think that this is because those 2 games where I won and lost 19lp was me duo queuing with a gold 4 friend. Is this a thing that I didn't know? Duo queue is taken into account when you win or lose a ranked match?